It is known to pressurize a liquid produced by a distillation column in a pump and to vaporize it in the main exchanger of the apparatus in order to deliver pressurized gas. The gases produced are sometimes contaminated with the barrier gas from the pump. The barrier gas used most frequently is nitrogen since it is available in the separation units and has a low cost price and a suitable pressure (see Epifanova and Axelrod, Separation de l'air [Air separation], Editions Mashinostroienie 1964, Volume 1, pages 199, 203). If the pumped liquid is oxygen, it may be contaminated by this nitrogen because of the wear of the labyrinth seals of the pump or because of a maladjustment of the control members, or by back-diffusion.
To alleviate this problem, JP 49,117,364 proposes to vaporize a portion of the liquid intended for the pump in an exchanger dedicated to producing the barrier gas.
One aim of the invention is to simplify the plant.
One object of the invention is to provide a process for separating a gas mixture by cryogenic distillation, in which:
at least one portion of the mixture compressed and purified in an exchanger is cooled; PA1 the cooled mixture is sent to a distillation column of a system comprising at least one column; PA1 a liquid is withdrawn from a column of the system; PA1 the liquid is pressurized in a first pump; PA1 use is made, as barrier gas for one or more pumps of the system, of: PA1 a portion of the liquid pressurized in the first pump is vaporized in the exchanger where at least one portion of the gas mixture cools; PA1 at least one gas produced by the column is heated in the exchanger where at least one portion of the gas mixture cools; PA1 only the portion of the gas mixture and the liquid which is pressurized in the first pump are sent to the exchanger; PA1 the liquid vaporized in order to form the barrier gas is pressurized by a second pump, this liquid pressurized in the second pump having the same main component as the liquid pressurized in the first pump and being as pure as or purer than the liquid pressurized in the first pump; PA1 the gas mixture is air and the liquid pressurized in the first pump is rich in oxygen, nitrogen or argon; PA1 the gas mixture contains at least 1% methane, at least 1% carbon monoxide, at least 1% hydrogen and/or at least 1% nitrogen and the liquid pressurized in the first pump is rich in methane, carbon monoxide, hydrogen or nitrogen; PA1 the barrier gas is at a pressure greater than the suction pressure in the first pump; PA1 the barrier gas is sent to all the pumps of the system which pump a liquid having the same main component as the liquid pumped by the first pump; PA1 the system includes a pump for sending a liquid from one of the columns to the storage tank and/or a falling-film-evaporator pump and/or a pump for sending liquid from one column of the system to another column of the system and in which the barrier gas for at least one of these pumps is PA1 a heat exchanger, PA1 a first pump and possibly a second pump, PA1 a system of distillation columns, including at least one column, PA1 means for sending at least one portion of the gas mixture to the heat exchanger and then to a column of the system, PA1 means for withdrawing a liquid from a column of the system and for sending it to one or more pumps and PA1 means for sending the liquid from the first and/or from the second pump to the exchanger and means for withdrawing the vaporized liquid from the exchanger; PA1 another heat exchanger in which the liquid intended to deliver the barrier gas is vaporized; PA1 the gas mixture is air and the system of columns comprises a triple column, a double column or a single column, possibly combined with an argon column and/or a mixing column; PA1 the first pump and possibly the second pump is (are) (a) labyrinth-seal pump(s); PA1 means for sending the same barrier gas to all the pumps of the system which pressurize a liquid having the same main component as the liquid pressurized by the first pump; PA1 a pump for sending a liquid from a column of the system to the storage tank and/or a falling-film-evaporator pump and/or a pump for sending liquid from a column or a condenser of the system to another column or another condenser of the system; PA1 the means for sending a barrier gas for at least one of the pumps comprises:
i) a gas coming from an external source other than a column of the system and/or PA2 ii) the vapour produced by vaporizing a liquid coming from an external source and/or PA2 iii) the vapour produced by vaporizing a portion of a liquid withdrawn from a column of the system and pressurized in the first pump or a second pump. PA2 i) a gas coming from an external source other than a column of the system and/or PA2 ii) the vapour produced by vaporizing a liquid coming from an external source and/or PA2 iii) the vapour produced by vaporizing a portion of a liquid withdrawn from a column of the system and pressurized in the first pump or a second pump. PA2 i) at least one external source of a gas other than a column of the system and means for sending this pressurized gas to the first pump in order to serve as barrier gas and/or PA2 ii) at least one external source of liquid other than a column of the system and means for vaporizing at least one portion of this liquid and for sending the vapour thus formed to the first pump in order to serve as barrier gas and/or PA2 iii) means for withdrawing a liquid from a column of the system and for vaporizing at least one portion thereof downstream of the first or of the second pump in order to deliver a barrier gas for the first pump. PA2 i) an external source of a gas other than a column of the system and means for sending this pressurized gas to the first pump in order to serve as barrier gas and/or PA2 ii) an external source of liquid other than a column of the system and means for vaporizing at least one portion of this liquid and for sending the vapour thus formed to the first pump in order to serve as barrier gas and/or PA2 iii) means for withdrawing a liquid from a column of the system and for vaporizing at least one portion thereof downstream of the first or of the second pump in order to deliver a barrier gas for the first pump.
According to other optional aspects of the invention, provision is made so that:
Another object of the invention is to provide a plant for separating a gas mixture by cryogenic distillation, which comprises:
According to other optional aspects of the invention, the plant includes: